Form

<form name = "form1">
            <label>Username</label><br>
            <input id="username" name="username" type="text" minlength="5" maxlength="8" required ><br>
            <label>First Name</label><br>
            <input id="firstName" type="text" required><br>
            <label>Last Name</label><br>
            <input id="lastName" type="text" required><br>
            <label>E-mail</label><br>
            <input id="email" type="text" required><br>
            <label>Password</label><br>
            <input id="password" type="password" minlength="7" required><br>
            <label>Mobile Number</label><br>
            <input id="mobileNumber" type="tel" pattern="[0]{1}[1]{1}[0-9]{9}" required><br>
            <input type="button" value="Submit" id="submit" onclick="validations()"/>
        </form>
        <script>
            var email = document.getElementById("email");
            var username = document.getElementById("username");
            var firstName = document.getElementById("firstName");
            var lastName = document.getElementById("lastName");
            var reg = /^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{​​​​2,4}​​​​$/;
            var txt = /^[a-zA-z]+$/;
            var password = document.getElementById("password");
            function validations(){
                //empty validations
                if(username.value == "" || email == "" || firstName.value == "" || lastName.value == "" || password.value == ""){
                    alert("Error! Empty field(s)")
                }
                if(username.textLength < 5 || username.textLength > 8){
                    alert("Error! Invalid username")
                    console.log(!txt.test(un.value))
                }
                else if(reg.test(email.value)){
                    alert("Error! Invalid email")
                    console.log(!txt.test(em.value))
                }
                else if(password.textLength < 8){
                    alert("Error! Invalid password")
                    console.log(password);
                }
                else if(!txt.test(firstName.value)){
                    alert("Error! Invalid first name")
                    console.log(!txt.test(firstName.value))
                }
                else if(!txt.test(lastName.value)){
                    alert("Error! Invalid first name")
                    console.log(!txt.test(lastName.value))
                }
                else{
                    alert("Welcome!")
                }
            }
        </script>